Field of the Invention
This invention relates to an amplifier, especially to an amplifier circuit applied to a source driver of a display apparatus.
Description of the Related Art
Please refer to FIG. 1. FIG. 1 illustrates a schematic diagram of a conventional amplifier circuit applied in a source driver of a liquid crystal display. As shown in FIG. 1, the conventional amplifier circuit 1 applied in the source driver of the liquid crystal display is usually formed by a two-stage operational amplifying circuit 10 and an electrostatic discharge (ESD) protection circuit 12.
In fact, the two stages of the two-stage operational amplifying circuit 10 are a high-gain operational amplifier 100 and an output stage 102 having high driving capability. The output stage 102 is formed by two MOS transistors M1 and M2. Since the output terminal OUT of the amplifier circuit 1 is coupled to output pads, it is possible that the output terminal OUT of the amplifier circuit 1 receives the static electricity; therefore, the ESD protection circuit 12 is disposed in the amplifier circuit 1 to protect the two-stage operational amplifying circuit 10 from being damaged by the static electricity.
As shown in FIG. 1, the ESD protection circuit 12 of the amplifier circuit 1 is usually formed by two bipolar junction transistors (BJTs) 120 and 122 coupled in series. The PNP BJT 120 is coupled to the power source terminal SUP to form a PN diode; the NPN BJT 122 is coupled to the ground terminal GND to form another PN diode. Once the output terminal OUT of the amplifier circuit 1 receives the static electricity, the above-mentioned PN diodes will guide the static electricity to the power source terminal SUP or the ground terminal GND to avoid the damage caused by the static electricity flowing into the two-stage operational amplifying circuit 10.
In practical applications, if the output stage OUT and the negative input terminal INN of the two-stage operational amplifying circuit 10 are directly coupled to form a negative feedback or coupled through the passive component (e.g., a resistor), it can be used as a buffer. At this time, under the imaginary short effect of the amplifier, the output voltage at the output terminal OUT of the two-stage operational amplifying circuit 10 is equal or similar to the input voltage received by the positive input terminal INP of the two-stage operational amplifying circuit 10. In the applications of the source driver of the LCD apparatus, it is necessary to use the two-stage operational amplifying circuit 10 as the buffer to smoothly drive the equivalent multiple stages RC series load terminal of the LCD apparatus.
The amplifier circuit in the above-mentioned prior art have the following drawbacks of: (1) the layout area of the ESD protection circuit 12 and the output stage 102 of the two-stage operational amplifying circuit 10 are too larger, so that the cost of the amplifier circuit 1 is high and the area is hard to be reduced; (2) the slew rate of the amplifier circuit 1 caused by the parasitic capacitance generated by the diode circuit and the metal cable in the ESD protection circuit 12, so that the dynamic power consumption and the temperature of the amplifier circuit are increased.